Fuel cells have been used as a power source in many applications. Fuel cells have also been proposed for use in electrical vehicular power plants to replace internal combustion engines. Typically, fuel cells are stacked or arranged so as to provide a single supply power. However, most electric-powered vehicles require an operating voltage which is greater than the supply voltage provided by most fuel cell stacks. As a result, a DC/DC boost converter is needed to increase or boost the voltage from the fuel cell stack up to the required operating voltage level.
Known DC/DC converters used in such fuel cell applications typically include an inductor and a switching means. To date, the inductor has been designed such that the inductance remains relatively constant as the current through the inductor varies from a low load to high load condition. This type of inductor is commonly referred to as a linear inductor. FIG. 1 is a graph illustrating a substantially linear rate of inductance versus current for a typical known linear inductor used in DC/DC converters.
The linear inductor includes a first terminal for receiving supply power from the fuel cell stack and a second terminal connected to ground by way of the switching means. The switching means has an open position and a closed position. In the closed position, the switching means creates an electrical path between the second terminal of the inductor and ground. In the open position, the switching means opens the electrical path between the second terminal of the inductor and ground or, in other words, creates an open circuit. The switching means is switched or toggled between the open and closed positions at a switching frequency to alternately produce and collapse a magnetic inductance field about the inductor and charge an output capacitor. In this manner, the converter provides an increased output voltage.
In known DC/DC converters which include a linear inductor, repetitively switching or toggling the switching means between the open and closed positions produces an AC ripple current. Ripple current is an increase in current draw, relative to a nominal current draw, upon the fuel cell stack when the switching means is closed. The linear inductor typically used in DC/DC converters produces a constant AC ripple current during both low and high load conditions. Ripple current detrimentally effects the fuel cell stack by increasing the effective or RMS current drawn from the fuel cell, thereby, increasing ohmic losses in the fuel cell stack and decreasing fuel cell efficiency. Typically, the amount of voltage produced by a fuel cell is used as a measure of the efficiency of that fuel cell.
One way to reduce or limit the effect of ripple current is to increase the switching frequency of the switching means. However, a higher switching frequency results in increased switching losses in the semiconductors (i.e. the transistors and diodes) within the converter. Accordingly, it would be desirable to provide a DC/DC converter which overcomes the shortcomings of the prior art.